Bleach (sodium hypochlorite) is a commodity chemical that is used in numerous applications. The basic chemistry for manufacturing bleach is a matter of common knowledge in the fields of chemistry and chemical engineering. Chlorine in gas and/or liquid phase is allowed to react with a solution of sodium hydroxide (caustic) to yield aqueous sodium hypochlorite. While that basic chemistry may be considered rather elementary, and essentially common to all processes for the commercial manufacture of bleach, specific processes that have been described in patent literature differ in significant ways.
Each of the various known processes for the commercial manufacture of bleach may be characterized as either a batch (discontinuous) production process or a continuous production process. Each type of process may have its own particular advantages.
A continuous process that is properly controlled is more likely to be performed with higher production efficiency than a corresponding batch process, and hence is likely to be more economical than a batch process. However, the specific manner in which a continuous process is performed plays a significant role in the nature and quality of the resulting bleach product.
U.S. Pat. Nos. 4,428,918 and 4,780,303 each describes a respective continuous process for manufacture of concentrated (i.e. high-strength) sodium hypochlorite solutions. Sodium chloride (salt) is however also a product of the basic reaction, and its removal from the aqueous sodium hypochlorite product can improve both the continuous process and the resulting product. Neither of those processes removes all of the salt from the resulting product.
Both patents recognize that certain batch processes can produce aqueous high-strength bleach from which significant amounts of salt have been removed.
It is believed that a continuous process that is capable of consistently producing aqueous high-strength bleach with low concentrations of both sodium chloride and sodium chlorate with residual slight excess caustic would be beneficial to industry. A product that has even greater strength, and lower salt and chlorate concentrations, than those mentioned in U.S. Pat. Nos. 4,428,918 and 4,780,303 would be especially beneficial. The benefits reside both in the utility of the product and relevant economic factors.
The process that is the subject of the inventors' priority provisional and non-provisional patent applications initially creates, and then continuously replenishes, a salt slurry in a bottom zone of a crystallizer stage tank. As fresh solutions of bleach and caustic continuously enter into solution in the tank, the slurry at the bottom is being continuously pumped out.
A first portion of the withdrawn slurry forms a recycle solution that is cooled during passage through a heat exchanger before being fed back into the tank. The fresh caustic is entrained with the recycle solution ahead of the heat exchanger. Fresh bleach is entrained with the entrained caustic and recycle solution after the heat exchanger.
The crystallizer stage tank shown in the priority patent applications comprises a skirt baffle that is inside the cylindrical sidewall of the tank and forms a cylindrical wall to create an annular calming zone between the skirt baffle and the tank sidewall. The annular calming zone is essentially free of turbulence, especially toward the top where an upper zone of essentially crystal-free mother liquor is created. The skirt baffle surrounds a central inner zone into which the fresh bleach and caustic and the recycle solution are introduced. The annular calming zone and the central inner zone are both above and open to the bottom zone.
Continually overflowing mother liquor at an appropriate rate from the top of the calming zone supersaturates the solution resulting in salt continuously precipitating out of solution with the salt crystals continuously replenishing the slurry in the bottom zone.